What is Corona in transmission lines and its occurrence

In power system engineering, we see various transmission and distribution losses occurring in the transmission lines. In India, the Transmission and Distribution losses amount to 35% which is one of the highest in the world. Corona loss is one of the T&D losses which occur in a transmission line.

In this post I want to tell you what exactly is corona and what is responsible behind the occurrence of corona.

In transmission lines, you might have seen 2 conductor cables at certain predefined distance to each other: one phase and other neutral. When the potential difference between the 2 cables, increases beyond a certain value called Critical Disruptive Voltage (usually beyond 30kV), then the cables start glowing with a violet glow. This phenomenon is said to be the occurrence of corona.

This violet glow usually occurs along with a hissing sound, power loss and radio interference. It is also responsible for producing ozone gas.

Animation of Electric Discharge (Corona Discharge is quite similar to this process)

This violet glow is more uniform when the transmission conductor cables are polished smoothly otherwise, the violet glow is more at the rough surface of the conductor.

Let me tell you the physics behind this process...

In between the overhead conductors, air is present which is itself composed on ions. So when the potential difference between the conductors exceeds say 30kV, then the ions get charged and start moving from higher potential to lower potential region across a potential gradient. These charged ions or say electrons collide with the particles of the conductors to eject electrons from its surface. This ejected electrons when seen as a huge mass appears to be violet glow over the surface of the conductor.